A longer moving stroke, a higher output (a larger driving force), and a smaller size are required for a linear solenoid. The linear solenoid is known in the art, for example, as disclosed in Japanese Patent Publication No. 2006-46627, Japanese Patent Publication No. H9-144931, and Japanese Patent No. 3601554.
A linear solenoid of a related art will be explained with reference to FIG. 10.
The linear solenoid is composed of an electromagnetic coil 1 for generating electromagnetic force, a plunger 2 as a moving member, a magnetic pulling portion 6 for magnetically pulling one end of the plunger 2 to one of axial ends (a left-hand side in FIG. 10), and a magnetic path portion 11 through which magnetic flux passes in a radial direction of the plunger 2.
The plunger 2 is biased toward the other axial end (a right-hand side in FIG. 10) by a biasing member, such as a spring. Electromagnetic attracting force is generated at a main gap between the plunger 2 and the magnetic pulling portion 6 by electromagnetic force generated at the electromagnetic coil 1, so that the plunger 2 is moved toward the one axial end.
As explained above, higher output and further miniaturization are always required for the linear solenoid. The electromagnetic attracting force for attracting the plunger 2 in the axial direction is generated only at one axial end of the plunger 2 (that is, at the main gap). In addition, an outer diameter of the plunger 2 at the main gap (i.e. an electromagnetic force generating portion) is smaller than a minimum inner diameter of a bobbin 5 for the electromagnetic coil 1. Therefore, an outer peripheral length of the plunger 2 at a portion for receiving the electromagnetic attracting force is relatively short.
As a result, magnetic saturation may easily occur at the portion for receiving the electromagnetic attracting force in the above explained linear solenoid. It is, therefore, difficult to realize larger electromagnetic attracting force for the linear solenoid having a longer moving stroke of the plunger 2.